It is known to incorporate elastic-type elements into a continuous web in a continuous processing operation wherein the elastic elements extend in the longitudinal, or machine, direction in the web. Such elastic elements may be incorporated in such manner that the elements are in a stretched state, having retractive forces active in the longitudinal, or machine, direction, during the continuous processing of the web. So long as such retractive forces are confined to the machine direction, the effect of the retractive forces on the registration of the web can be controlled by controlling the tension on the web.
In continuous processing of a substantially endless web, as in the instant invention, it is known to perform a plurality of processing steps on the web in sequence. Typically, it is desired to perform one or more of the processing steps at specific locations in the web, wherein a second step is performed at a particular locus with respect to a first, or previous, step, whereby it is critical to be able to locate, at the work station for performing the second step, the location at the which the previous step was performed. In the art, this is known as being able to "register" the second step with respect to the first step.
A first factor in being able to register a second processing step with respect to a first processing step, is being able to determine the distance, and the direction, that the work locus on the web where the first step was performed, has travelled from the location in the processing apparatus where the first step was performed. Such determination can be divided into four assessment steps.
First, one determines the longitudinal distance, in the machine direction, travelled by the work locus in the web.
Second one determines the width distance, in the cross-machine direction, travelled by the work locus.
Third, one determines shrinkage in the machine direction.
Fourth, one determines shrinkage in the cross-machine direction.
The first and third factors can be measured and controlled using conventional control apparatus and methods to control drive speed and tension in the web. The second factor can be controlled using conventional lateral guide apparatus and methods.
The fourth factor is normally accommodated as a dependent dimension controlled by the amount of tension/tensile stress imparted to the web in the machine direction. Namely, as tensile stress is applied to the web in the machine direction, the amount of strain/stretch in the machine direction produces a dependent amount of shrinkage in the web in the cross-machine direction.
Such accommodation, however, assumes that no retractive-type forces are active in the web in the cross-machine direction. To the extent retractive-type forces are active in the web in the cross-machine direction, absent other controls, the web shrinks in the width (cross-machine) direction in response to such active retractive-type forces, thereby changing the width of the web in the cross-machine direction. In processing operations where it is important to maintain a constant, or nearly constant width in the web, other means are needed to maintain constant the width of the web.
It is known to maintain the width of the web using a tenter frame, wherein an assemblage of gripping devices grips the opposing edges of the web at closely spaced locations, and travels with the web, thus physically restraining the web against reduction in the width dimension so long as the gripping devices have hold of the edges of the web.
Use of tenter frames have four severe limitations. First, they occupy precious working space adjacent the web being processed such that they may interfere with set-up and/or operation of the processing steps where by the web is otherwise modified.
Second, tenter frames are relatively complex, expensive mechanical devices.
Third, use of tenter frames is generally limited to straight stretches of the processing line. Namely, tenter frames generally are not used to maintain the width of the web as the web traverses about the circumference of a turning roll or a processing roll in the sequence of steps in the processing line. Yet turning rolls and processing rolls are commonly preferred as efficient rotary devices for performing routine steps on webs being processed.
Fourth, the edges of the web gripped by the tenter frame grippers are generally not subsequently used as part of the finished product, whereby the grippers may make a positive contribution to the amount of waste associated with the manufacturing operation.
Another known method for urging width-dimensional stability on a web under cross-machine direction tension is to process the web on a roll coated with silicone rubber, as taught in, for example U.S. Pat. No. 4,925,520 Beaudoin et al. However, a silicone rubber coating presents a number of limitations related to the characteristics of silicone rubber. For example, silicone rubber on a roll carrying the web cannot provide sufficiently rigid surface against which to perform slitting and like cutting operations.
Thus, it would be desirable to provide additional methods and apparatus for controlling the dimensional stability of a web in the cross-machine direction wherein the web incorporates therein retractive-type forces active in the cross-machine direction.
It is an object of this invention to provide apparatus and methods for providing cross-machine dimensional stability in a continuous web being processed in a continuous process, the web having stretched elastic-type elements which exhibit retractive-type forces active in the cross-machine direction.
It is a further object to provide such apparatus and methods, including a first rotary transport device, transporting the web, including a first set of protuberances thereon for engaging and holding the web, and transferring the web to a second rotary transport device including a second set of protuberances thereon for engaging and holding the web.
It is a further object to provide such apparatus and methods wherein the first and second rotary transport devices have outer working surfaces, carrying the web, which outer working surfaces are aligned with each other across the entire width of the web.
It is yet another object to provide such apparatus and methods wherein the composition of one or more of the outer working surfaces comprises an outer coating incorporating a release agent such as polytetrafluoroethylene.
It is still another object to provide apparatus and methods including suction into one or both of the rotary transport devices.
Yet another object is to provide apparatus and methods wherein the width direction shrinkage of the web is limited to no more than about 5%, preferably no more than about 1%, most preferably no more than about 0.5%.